Engine failures...Best glide or best rate of sink

[flyalot]

Go best glide. You always come down. But you can't "streeeeeetch" the glide. Yes, speaking from exeirence.

 

[avbug]

You use what you feel you need. Blindly pitching to a particular speed may not be in your best interests. I don't know that waiting until you experience an engine failure is the best time to be calculating anything. You should already know what you are going to do...and you should know this firmly before you every apply power to take off.

Best glide speed gets you maximum forward distance for a given altitude loss. If you're trying to get somewhere, then this is your best performance for getting there. However, if you're not trying to get somewhere (as is often the case in an engine failure, because let's face it...you're not going anywhere), then best glide isn't necessarily in your best interest.

If you want maximum time aloft, minimum sink is in your best interest. This may apply to preparing for a forced landing, restarting an engine, communicating for emergency purposes, or establishing the minimum sink possible for landing on water or unknown/obscured terrain. Minimum sink speed provides your minimum descent and minimum forward speed in a glide; it's the best you're going to do in a steady-state descent to minimize impact forces. (Not a great comfort when you're flying an empty beer-can as most airplanes are wont to be...but it's the best you can do).

Minimum sink isn't stall speed.

Initially going for your best glide speed is a good reaction, and for the most part, a safe one. If you're in the habit of doing that, you're engaged in a safe habit. From there, you can always adjust to fit your needs. My last failure occured at 150', and I was able to gain a total of about 300' altitude before completely losing the engine. I ended up reducing speed slightly below best glide when clearing the last set of powerlines before setting down on a hillside. Just prior to touchdown, I deployed full flaps in order to use them to absorb impact and lessen final touchdown speed. I touched down at minimum sink. I needed to stretch the glide, especially as I didn't have much to work with from the start. I had very little time to make a choice, and very little choice. My choices had been made earlier in the day when I surveyed the area, and had been an on-going process throughout the afternoon...which is the way it should be...your first thought at the time of failue shouldn't be what to do...you should already know.

This is especially true of your airspeed and aircraft control. Know your proceedures. A lot of engine failures are fuel related...you may stand a very good chance of having a restart, if you follow the correct proceedures, and your best bet for doing this is having the aircraft under control and trimmed up to enable you to reference your checklist and execute the proceedures. Starting by trimming for best glide is a good choice. If you're over a suitable forced landing site (which you should be anyway), then minimum sink offers you the maximum time to do this.

Minimum sink also can become a defeatist mindset, in that if you attempt to fly this airspeed and dedicate yourself to it, you may be in a denial state of mind. Delay ground contact and impact/landing as long as possible. On the one hand you do want this, but on the other, you know you're going to be getting there eventually; you are going to be reaching the ground at some point. Don't fly minimum sink with your nose in a checklist all the way to impact...at some point you need to abandon restart efforts when they've failed and concentrate on executing that forced landing like your life depends on it. Continuing to maintain minimum sink isn't a bad thing, so long as you're heads up and flying the airplane.

As a very general rule of thumb, best glide speed approximates Vy at sea level, and minimum sink approximates Vx at sea level. Vx and Vy change with altitude in a normally aspirated engine...they come together. Your best glide speeds, for your purposes, do not. Therefore, you'll note that your best glide speed is very close to your published sea level best climb speed. Many aircraft don't publish a minimum sink speed but it's going to be very close to your published best angle of climb speed, or Vx.

You can try this in flight at idle...try gliding descents at speeds starting with the published best glide, and take notes with each glide. Decrease your glide speed for each attempt by five knots, and note your descent rate each time. You'll quickly zero in on a minimum descent rate, and you'll find it's occuring at an airspeed close to Vx. Be sure that when conducting this experiment you give the airplane ample time to stabilize at each airspeed and conduct the descents from the same altitude. Start at say, eight thousand, and set up a glide for a thousand feet or two thousand feet at 70 knots. Then do it from eight thousand feet at 65 knots, and climb back to eight thousand to do it at 60 knots and so on. If at 65 knots you see less sink average sink rate than at 60 or 70, you can start refining your experiment by using smaller increments than 5 knots, if you like. See what your airplane will do.

I've had a number of engine failures. More partial power failures than complete failures though a number of the partial failures required the engine to be shut down. Most resulted in landings at an airport, some off field. None of mine to date have been fuel exhaustion, and most have been mechanical. I have never tried to calculate my glide distance or run charts to calculate peformance during an engine failure. I'm not sure what good it would do. You should have an idea of how far you can go as a reference from the cockpit...looking over the nose, for example, you might know that you can reach whatever you see directly past the cowl, or directly past your landing gear or whatever applies to your specific airplane...you should have an idea how far you can go. You should also not plan on stretching it. That's a little like passing fuel stops when a perfectly good fuel stop is right beneath you.

If you stay in the practice of keeping landing sites beneath you when you fly, or within immediate gliding distance, then you're a smart planner. That's good airmanship. Following roads in mountainous terrain, keeping valleys with floors or medows capable of being landed on, staying close to the shore when over water, and other basic practices of safety, are good ideas, and minimize your having to think about how far to stretch your glide. Don't get caught in the idea that you're going to glide to an airport, either. You might be able to do that, but most of the time, you won't. You might have a partial power failure and you're not sure if the airplane is going to make it...if you can get over a freeway or landing surface and follow that to the airport, fine...but don't give up a perfectly good opportunity to put the airplane down in favor of trying to eek on over to a runway somewhere. You might have give up your last chance.

Best glide vs. minimum sink...all depends on what you need at the time. Like the old guy in the Indiana Jones movie said, choose wisely.